The present invention relates to a method for processing converted mode seismic data and, more particularly, to a method for extracting shear velocities from conventionally recorded compressional mode data and performing normal moveout correction and stacking of the acquired P-SV converted wave seismic data.
The use of compressional, or P, wave reflection data in geophysics analysis is well known. A typical seismic reflection prospecting system which produces compressional wave reflection data would be comprised of a compressional wave source located on the surface and geophones spaced along a line of exploration on the surface for measuring the vertical component of the ground motion caused by the reflected compressional wave. However, conventional P waves travelling through the subsurface also produce vertically-polarized shear, or SV, waves capable of being detected by the geophones when reflection of the generated compressional wave off a reflecting interface at other than a normal angle of incidence occurs. Thus, seismic sections produced by such compressional wave seismic exploration systems would contain two types of wave information which, if properly exploited, will yield useful information regarding the lithologic characteristics of the subsurface formation under investigation. In recent years, interest has been growing in obtaining shear wave information to provide useful information regarding the lithological characteristics of the subsurface formation under investigation. Such information, if properly obtained and exploited, can be utilized in conjunction with information obtained from compressional wave seismic section to provide a more detailed analysis of the characteristics of the subsurface formation. For example, compressional wave seismic sections can provide useful information on the compressibility of subsurface formations, while shear wave seismic sections can provide useful information on subsurface formation rigidity.
Several limitations related to the characteristics of the shear wave have, however, prevented the full exploitation of shear wave information. Shear wave seismic reflections are noisier than compressional wave seismic reflections, making proper interpretation difficult. Furthermore, detecting shear wave reflections is more difficult than detecting compressional wave reflections since shear wave reflections are typically of much lower amplitude than compressional wave reflections. Finally, the direct propagation of a shear wave into a subsurface formation to induce a shear wave reflection requires special transducers and additional steps over and above those required for obtaining compressional wave reflection data. This makes obtaining shear wave reflection data difficult, more costly and time-consuming.
While difficult to obtain, shear wave data can be very useful in the exploration for hydrocarbons. Hydrocarbon deposits which produce compressional wave reflection amplitude anomalies do not produce similar shear wave reflection amplitude anomalies. Such a result occurs because shear waves do not respond to any fluids and therefore do not produce different amplitude responses for gas, oil, and water. In contrast, compressional wave amplitude anomalies that are caused by anomalous lithologies such as salt, coal and hard streaks usually have equally anomalous shear wave behavior. Such an application of shear wave information has not been widely exploited, however, because most amplitude anomalies of interest occur offshore while shear wave seismic data can only be recorded onshore.
When both P-wave and S-wave velocity information are available from field measurements, the simple ratio Vp/Vs may provide an estimate of the lithology. Use of the ratio in seismic work has the advantage of not requiring density information. The shear wave velocity of a porous rock has been shown to be less sensitive to fluid saturants than the compressional wave velocity. Thus, observation of the ratio of the seismic velocities for waves which traverse a laterally varying zone of gas or oil saturation could produce an observable anomaly which is independent of the regional variation in compressional wave velocity.
The publication entitled A Field Investigation Comparing Conventional Compressional-Wave. Converted-Wave, and Horizontally-Polarized Shear Wave Reflections by John T. Bellatti reported the use of a seismic prospecting system capable of generating compressional waves into a subsurface formation and recording converted P-SV data reflected off subsurface reflecting interfaces. While only Bellatti is discussed in detail below, similar teachings may be found by reference to the publication entitled A P-SV Converted Wave Reflection Seismic Prospecting System by John E. Graves.
Bellatti discloses that for a flat layer converted P-SV wave, the travel time equation for the formation may be represented as follows: ##EQU2## where:
t.sub.x =travel time;
V.sub.p =compressional wave velocity;
V.sub.s =shear wave velocity
X.sub.p =source--CDP offset;
X.sub.s =CDP--receiver offset;
t.sub.po =vertical travel time of the P wave; and
t.sub.so =vertical travel time of the S wave.
Bellatti also discloses that the travel time may be represented in terms of a Vp/Vs ratio. More specifically, Bellatti provides that: EQU t.sub.x =(t.sub.po.sup.2 (1+K.sup.2)+X.sup.2 /(Vp.sup.2 /K)).sup.1/2( 2)
where:
K=Vp/Vs; and
t.sub.so =Kt.sub.po.
Bellatti, however, relied upon separate P, P-SV and SH exploration systems and data gathers for collection and processing of seismic data. Extraction of shear wave velocities from conventionally recorded compressional mode data and performing normal moveout correction and stacking of a shot record including both P and P-SV data where the correction and stack takes the extracted shear wave information into account was nowhere contemplated by Bellatti. Such multiple exploration and multiple processing techniques is both time consuming and expensive.
When seismic data is processed to attempt to extract shear wave velocity information from compressional wave information, numerous problems arise. The most serious problem associated with such types of processing is that such estimates are extremely sensitive to normal moveout problems. Correcting for time or velocity so that nearly perfect time alignments are produced from trace to trace for every reflector in the CDP gather is required to prevent the introduction of velocity errors.
When determining the appropriate stacking velocities for compressional to vertical shear seismic data, the most commonly performed technique estimates the compressional and shear velocities as a function of seismic recording time. From these velocities, an appropriate P-SV velocity time function is calculated. This function is used to perform normal moveout and is followed by stacking and display for subsequent analysis, event identification and correlation to standard compressional wave seismic data. Such techniques typically assume an NMO function of the form: EQU Tnmo=(x.sup.2 /Vpsv.sup.2 +Tpsv.sub.0.sup.2).sup.1/2 ( 3)
where:
Tpsv.sub.0 =two way travel time within the media;
Vpsv=modified velocity within the media estimated experimentally from P and SV estimates; and
x=source-receiver offset.
Such a technique does not permit easy identification of P-SV events for comparison with the standard P wave section.